You come to a Rolling Stones 50th anniversary gig expecting an event. But you don't necessarily come expecting eventfulness. Spontaneity is not on the set list when any band is working arenas, and that is compounded, like interest, for an outfit with the gimlet-eyed professionalism of the Stones.

But on the second of their two London gigs this week, something quite thrilling occurs – a frisson that goes beyond the enduring thrill of hearing the ancient tablets of the rock law played aloud by their inscribers.

A tall, red-headed woman comes onstage to wail the air-raid bits of Gimme Shelter. We knew Florence Welch was coming, and that she, like the Stones, is originally from "round here" (as Mick Jagger notes in a pointed drawl). And we know that Eric Clapton, and two former Stones – Bill Wyman (bass; in 1962, out 1993) and Mick Taylor (guitar; in 1969, out 1974) – will appear. But not that Welch would upset the smug, back-slappy nature of these guest spots by going off like a banshee firecracker.

Wearing heels, a dandy's crushed red velvet suit and a ruff, Welch goes mano-a-mano with Jagger, pushing him around as they howl, clavicle to clavicle, hair flying. Rather than the usual pairing in which a legend meets a fawning foil, it's more like watching two cockerels scrapping in a dusty yard. To say the whole exchange is sexually charged would be a bit icky, but Welch's display isn't just mannish bravado, either. "It's just a kiss away," they bawl, and she plants a smacker aggressively on Jagger's cheek. Somehow, Welch manages to slightly recalibrate the Stones's time-worn approach to gender relations. Tonight, by contrast, Honky Tonk Women comes with a graphic in which a 50-foot cartoon lady loses her bikini top, and the aircraft attacking a tower in New York (!) literally explode on her bare breasts.

If the surprises of this landmark gig are few and far between, this dance of thrill and recoil recurs again and again. For every time the Stones play something that sounds bejewelled and seductive – a baroque Paint It Black, say, or the disco-era gem Miss You, or (I Can't Get No) Satisfaction, cut last Sunday because of curfew – there are times when the motions are gone through with all the joie de vivre of loss adjusters, and the slogan "50 years and counting" comes to resemble more of a sentence than an anniversary. One More Shot, one of the new songs, reveals a band on autopilot, painting not with black, but with a kit, by-numbers, in their sleep.

It's Only Rock'n'Roll returns Bill Wyman to the line-up, ostensibly to give Darryl Jones, de facto Stones bassist since 1993, a toilet break. It's only rock'n'roll at its most staid and chummy. At the end, Keith Richards puts an affectionate paw on Wyman's shoulder, and Jagger teases him about his signature metal detector.

Mick Taylor fares slightly better. On paper, Midnight Rambler is the moment when a bescarfed Taylor – the lead guitarist whose fluent stylings defined the early-70s Stones – reclaims his rightful place in the pantheon. Actually, the lengthy blues goes on far too long and – despite three very arresting sets of licks from three very different Stones guitarists – never quite matches its billing. When the smuggest of all guitar heroes, Eric Clapton, turns up, though, you are forced to sit up and listen, against your lovingly cultivated prejudices. Champagne and Reefer is a Muddy Waters tune that Clapton, and every Stone, lights up for – not least Mick Jagger, who channels a little more than usual of the satyr of old, taking liberties with the words.

Everyone here is acutely aware that there are only so many more times that these Rolling Stones – Jagger, Richards, Charlie Watts and Ron Wood – will play The Last Time. The privilege of new forms such as rock is that their originators are still here to be witnessed, and there is a deep satisfaction to be had in watching the 20th century's equivalents of Chaucer or Palestrina or Thespis run through their material. But even the Stones are not immortals. Ramrod-straight and on point all night, drummer Watts looks ashen by the end, and zips up into a jacket as soon as he comes off his drumkit.

There is, though, no warm glow of indulgence coming off the stage during the final run of songs, but the red pulsation of Sympathy for the Devil, and the connivance of band and audience that danger is afoot. Never mind that the black cloak Jagger dons at the start makes him look a bit like an extra from Monsters Inc. There is still, against the odds, potent ju-ju in this band's arch, stylised flirtation with the dark stuff.